1. Field of the Invention
The present invention relates to a liquid crystal display and, particularly, to a liquid crystal display panel of the In-Plane Switching (IPS) system and a fabrication method of the same liquid crystal display panel.
2. Description of the Prior Art
With the increased demand of liquid crystal display, the demand of reduction of fabrication cost of the liquid crystal display panel is also requested. In order to reduce the fabrication cost of a pair of glass substrates used in the liquid crystal display panel, it is usual to fabricate a large size strip-shaped glass substrate and a plurality of liquid crystal display panels are cut apart from the large size strip-shaped glass substrate.
Since the liquid crystal display panel is usually rectangular, the cutting direction of the large size strip-shaped glass substrate (referred to as xe2x80x9craw glass substratexe2x80x9d, hereinafter) with which the largest number of glass substrates for liquid crystal panel can be cut apart from the raw glass substrate is primarily determined by the size of the raw glass substrate. For example, it is possible to cut apart six panel substrates each for a 14.1 inches liquid crystal display panel from a 600 mmxc3x97720 mm raw glass substrate. That is, by cutting the raw glass substrate along a shorter side thereof such that longer sides of the respective panel substrates become in parallel, two columns each including three panel substrates can be cut out from the raw glass substrate.
Since the respective raw glass substrates are continuously fabricated as long glass belts having a predetermined width, it is unavoidable that the surface condition and/or thickness of the raw glass substrate has the direction dependency in a width direction and a lengthwise direction thereof the raw glass substrate. Particularly, since a surface of a raw glass substrate fabricated by the slot downdraw process becomes in contact with a slit of a fabrication device, fine line-shaped irregularity extending along a lengthwise direction tends to occur in the surface thereof. Therefore, when the raw glass substrate is to be used for substrates of the liquid crystal display panel, the surface of the raw glass substrate has to be polished to remove the irregularity. However, it is difficult to remove the direction dependency of thickness throughout the whole raw glass substrate even if the fine irregularity can be removed by polishing.
On the other hand, although such fine line-shaped irregularity hardly occurs in a surface of a raw glass substrate fabricated by the fusion process, the latter process is the continuous fabrication method and, therefore, the thickness of the raw glass substrate varies in a direction perpendicular to a pulling direction thereof. Consequently, the direction dependency of the surface condition and the thickness of the raw glass substrate is unavoidable.
Therefore, when a liquid crystal display panel is to be constructed by arranging a TFT (Thin Film Transistor) substrate constructed by forming pixel electrodes and switching elements such as TFT""s on one of the rectangular substrates cut apart from the continuously fabricated raw glass substrate and a CF (Color Filter) substrate constructed by forming a color filter (CF) on another of the rectangular substrates in an opposing relation through a liquid crystal layer, spacer members and a sealing member, the direction dependencies of the opposing glass substrates are in the same direction.
When a longer side direction of a TFT substrate 21 is the same as a drawing direction of the raw glass substrate during a fabrication of the raw glass substrate as shown in, for example, in FIG. 1A, there is the direction dependency of thickness of the TFT substrate 21 in the shorter side direction thereof as exaggeratedly shown by a cross section thereof in FIG. 1B. Since the shorter side direction of a CF substrate 22 is the same as the drawing direction of the raw glass substrate similarly as shown in FIG. 2A, there is the direction dependency of thickness of the CF substrate 22 in the shorter side direction thereof as exaggeratedly shown by a cross section thereof in FIG. 2B.
Therefore, when the liquid crystal display panel 23 is constructed by arranging the pair of such rectangular glass substrates in the opposing relation such that the longer sides thereof become in parallel, thicker portions 24 and 34 of the TFT and CF substrates may be substantially overlapped and thinner portions 25 and 35 thereof are also overlapped, as shown in FIG. 3A and FIG. 3B showing a cross section thereof. In such case, a distance 28 between an area 26 in which the thicker portions are overlapped and an area 27 in which the thinner portions are overlapped substantially becomes a half of a distance between adjacent areas of the overlapped thicker portions.
In such case, a larger pressure than a pressure applied to the overlapped thinner portions is applied to the overlapped thicker portions in the pressing step for adhering the TFT substrate to the CF substrate under pressure of the fabrication process of the liquid crystal display panel 23. That is, a difference in pressure between the area 26 and the area 27 is provided within the liquid crystal display panel 23. In the area in which the larger pressure is applied, the spacers and the CF layer may be deformed more. The deformation includes elastic deformation and plastic deformation. Although the elastic deformation is removed when the pressure thereto is removed, the plastic deformation cannot be removed even when the pressure is removed. Therefore, in a portion having large plastic deformation, that is, the area 26 in which the thicker portions are overlapped, a gap between the TFT substrate 21 and the CF substrate 22 becomes small and, in the area 27 in which the thinner portions are overlapped, the gap becomes large. In other words, the variation of the thickness of the substrate produces a variation of pressure and hence a variation of the gap between the TFT substrate and the CF substrate. Since light transmittance of liquid crystal depends upon the gap between the TFT substrate 21 and the CF substrate 22, the variation of gap causes a variation of display. As such, a small variation of the display cell gap is unavoidable throughout the panel.
In a conventional liquid crystal display panel employing TN (Twisted Nematic) mode liquid crystal utilizing rotary polarization, the influence of small variation of the display cell gap on the variation of light transmittance of liquid crystal is small. However, since a liquid crystal display panel using IPS mode or VA (Vertical Alignment) mode, which is a wide view angle mode, utilizes the birefringence effect of liquid crystal, the influence of the variation of cell gap on the light transmittance of liquid crystal is large compared with the conventional liquid crystal display panel employing TN mode. Further, although there is a tendency of reducing the cell gap in order for increase of response speed of display, the reduction of cell gap makes light transmittance of liquid crystal sensitive to the variation of gap.
Therefore, the variation of display caused by variation of cell gap, which caused no problem in the past, becomes a problem recently. That is, it has been requested to control the display cell gap of the liquid crystal display panel uniform.
An object of the present invention is to provide a substrate arranging structure of, for example, a liquid crystal display device, for reducing the irregularity of display, which is caused by thickness distribution or irregularity distribution of opposing substrates used in the liquid crystal display device, and a fabrication method of the substrate arranging structure.
The present invention is featured by that a direction of thickness distribution or irregularity distribution of a TFT substrate becomes orthogonal to a direction of thickness or irregularity of a CF substrate when the substrates are laid one on the other during a fabrication process of a liquid crystal display panel.
The liquid crystal display panel according to the present invention comprises the first substrate, the second substrate arranged in an opposing relation to the first substrate and liquid crystal disposed in a gap between the first and second substrates, wherein the first substrate has a distribution of thickness or irregularity variation in one direction and the second substrate has a distribution of thickness or irregularity variation in one direction substantially orthogonal to the direction of distribution of thickness or irregularity variation of the first substrate. In more detail, the TFT substrate and the CF substrate are arranged in the opposing relation such that the direction of irregularity of the TFT substrate becomes orthogonal to the direction of irregularity of the CF substrate.
The fabrication method of a liquid crystal display device, according to the present invention, is featured by that the first and second substrates are cut apart from a raw glass substrate having direction dependency such that the direction dependency of the first substrate becomes orthogonal to the direction dependency of the second substrate. That is, the fabrication method comprises the step of cutting apart first rectangular substrates from a first raw glass substrate having line-shaped irregularity such that longer sides of the first substrates become in parallel to a drawing direction of the first raw glass substrate, the step of cutting apart rectangular second substrates from a second raw glass substrate having line-shaped irregularity such that long sides of the second substrates become orthogonal to a drawing direction of the second raw glass substrate and the step of arranging the first and second substrates in an opposing relation with the long sides of the first and second substrates become in the same direction, while maintaining a gap necessary to receive a liquid crystal layer therebetween.
When the first substrate is a TFT substrate having thin film transistors, the second substrate is a CF substrate having a color filter and it is preferable to construct a liquid crystal display panel of IPS system.